In producing or injecting hydrocarbons or other fluids within a subterranean formation from a well borehole, it is often necessary to treat the formation to increase its productivity. One well known technique for increasing productivity is to hydraulically fracture the formation, e.g., pumping a fracturing fluid down a wellbore and into the formation at a pressure above which the formation parts, which creates one or more channels (i.e., failures or fractures) in the formation through which fluids can easily flow. In some of these methods, a proppant (e.g., sand) is included with the fracturing fluid to keep the fracture open after the formation fracturing pressure is reduced (and bedding planes tend to come together).
A single fracture (e.g., a single bedding plane separation emanating in both directions from a well borehole) would be normally produced by hydraulic fracturing methods. The single fracture is at a weak discontinuity (e.g., between sedimentary layers) or perpendicular to the direction of the principal stress. These single fractures increase productivity, but generally do not interconnect with other fractures to reach portions of the formation away from the single plane, leaving large, potentially productive zones unconnected to the borehole. If a reliable method of hydraulic fracturing could produce a multiplicity of deep fractures in directions radiating from the borehole, a significant increase in hydrocarbon fluid production may be possible.
In a common process, the fracture fluid is supplied from surface equipment, e.g., high pressure pumps, through high pressure tubing to the formation of interest, which may be isolated by packers. The high pressure tubing avoids excessive pressures/damage to casing, cement or formation at areas other than the formation of interest. When the surface pumps are actuated, the pressure increases at a rate determined by the pumping equipment. Once the initial fracture is initiated, the fluid pumping must be at rates sufficient to open and extend the fracture (and emplace proppant, if used). Because of slow loading rates and low pressures, usually only one fracture is formed during this type of hydraulic fracturing process.
Fracture fluids, such as high viscosity liquids, can be selected to decrease fluid (and pressure) loss at the fracture(s). However, high viscosity results in other problems, such as increased frictional pressure loss in the tubing. Cross-over ports and other methods have also been used to mitigate the fluid flow/pressure limitations inherent in surface pumps and tubing, but with limited success.
Another approach to these limitations on the number of fractures is to seal off initial fractures, temporarily limiting fluid and pressure loss through these fractures during the hydraulic fracturing process. This has been done by packers, entrained ball sealers, and sand plugbacks. However, these temporary blockage approaches add material costs, equipment costs, time and risk.
Another method of obtaining multiple fractures is to use an explosive charge or explosive perforation of the casing. The very rapid duration of the explosive effects cause multiple, but shallow fractures and undesirable pulverization of formation rock.
The heat and explosive nature of the charges can damage the casing, cement, or formation in areas where fractures are unwanted. Still further, the fractures created are not propped open (insufficient time to carry proppant to all fractures). Thus, quickly after the pressure decreases, the fractures may close and not form highly conductive paths from the well borehole to deep within the formation.
In addition, because of the nature of the explosion, the peak absolute pressure and loading rates may be poorly controlled. This may cause more damage (for uncontrolled high values) or an inability to open sufficient fractures (for uncontrolled low values).
A more recent method of producing a multiplicity of fractures uses an in-situ combustion process (e.g., rocket propellant and oxidizer) to generate a tailored pressure pulse. The combustion generates large volumes of gases downhole over short (e.g., up to tens of milliseconds), but not explosive time periods. The gas generation results in a rapid (but not explosive) pressure rise rate. The pressure rise rate is in between surface pumping rate limitations (generally less than 1 MPa/s) and rates from an explosive charge (generally greater than 10.sup.7 MPa/s).
Careful handling, however, similar to explosive handling, is needed for the propellants. Once the propellant is ignited, little control is possible. Propellant charges are also difficult to adapt to different applications.
A more economic, controlled and reliable means and method to obtain tailored pulse loading produced multiple fractures is needed. The device and method should also be capable of adapting to different applications. A minimum of effort to convert from one application to another is also desirable.